High-frequency heating



MY 8, 1951 E. MITTELMANN 2,551,757

HIGH-FREQUENCY HEATING Filed Aug. l0, 1945 INVENTOR. FJ

Patented May 8, 1951 UNITED STATES PATENT OFFICE 4 Claims.

This invention relates to a method and apparatus for heating by means of high frequency electric current.

The object of the invention is to secure optimum transfer of energy from a high frequency generator to the material or object to be heated. My invention is concerned. with the method and means for adjusting a variable coupling between an oscillator generator and a reactive heater to match initially the heater load to the oscillator/ and to govern the maintenance of the matched condition by the direct current component of the plate current drawn by the oscillator.

Another object of the invention is to provide high frequency heating apparatus and a method of operating that apparatus by means of which the apparatus may be pre-adjusted before the object or material is inserted in the reactive heater to establish a matched condition when the material or object is inserted in the heater and then to maintain automatically the matched condition during the entire heating operation. This is accomplished by a control which is calibrated at the factory, and operationally adjusted in terms of power level to cause automatic adjustment of a variable coupling between the oscillator and the reactive heater to initially match the heater to the generator when the material or object is inserted in the heater and to maintain the matched condition during the heating operation.

A still further object of the invention is to couple a high frequency oscillator to a reactive heater so that a wide change in the coupling ratio may be effected without danger of flash over. This is accomplished with the provision of one or more pairs of metal plates adjustable relative to each other so that the spacing between the plates increases with increasing Voltage across the plates.

Other objects and advantages will become apparent by reference to the following description in conjunction with the accompanying drawing wherein Figure 1 is a block diagram of a heating lapparatus including an electronic. oscillation generator and a reactive heater and provided with means embodying my invention for initially matching the load impedance to the generator impedance and for maintaining the matched condition during heating; and

Figure 2 is a schematic diagram of the apparatus of Figure l illustrating certain specific circuit details.

|The block diagram of Figure 1 shows avariable power oscillator G provided with a power level adjustor GI and connected by suitable power conductors to a variable coupler C which in turn is connected to a reactive heater H. A galvanometer W of a high frequency wattmeter is connected across a wattmeter sensitivity control resistor or potentiometer Wl which is in turn coupled through capacitors Cl and C2 to the transmission conductors interconnecting the variable power oscillator G and the variable coupler C. The variable power oscillator G is supplied with power from a three-phase full wave rectifier Rl energized from polyphase alternating current lines LI, L2 and L3.

A switch R3 is interposed in one of the conductors between the variable power oscillator G and the three-phase full wave rectifier Rl. ln the other conductor between the oscillator G and the rectifier Rl there is connected a resistor P which is by-passed by a capacitor C3 and is connected by a cable K to a calibrated potentiometer CP. The calibrated potentiometer CP is connected to control la voltage responsive bridge amplifier V which receives its power from a singlephase full wave rectier R2 energized from single phase alternating current lines L4 and L5 which may be supplied from a pair of the three-phase lines'. The bridge amplifier V controls a followup control F which is mechanically connected by a mechanism S to the variable coupler C.

The variable coupler C which includes spaced capacitor plates is adjusted through the mechanism S connected to the motor and the followup control F. The follow-up control F is responsive to unbalance of the bridge amplifier V which is so connected as to be governed by the oscillator anode or plate current, as reflected by the voltage drop appearing across the resistor P.

I have found that if the oscillator tubes are operated at each power level setting in such a manner that the plate current is driven into the saturation region of their plate voltage, plate current curves, then for matched conditions the plate current is such a. function of the power level setting of the oscillator and of the equivalent load resistance across the oscillator tank circuit that it may be used to govern the setting of the variable coupler in order to establish and maintain an equality between that resistance and the equivalent internal dynamic resistance of the oscillator, i. e., the optimum load resistance value of the oscillator at each power level setting.

Hence the potentiometer CP, bridge amplifier V, follow-up control F and variable coupler C are S0 calibrated at the factory as to establish initially a given plate current for each power level setting of the oscillator and to maintain that given value throughout the heating operation. The potentiometer CP is calibrated for different power level settings ci the oscillator G, its dial being preferably calibrated in the same terms as the power level adjuster Gl. rlhe resistor P is adjusted or calibrated at the factory to obtain the desired degree of sensitivity in the response of the bridge amplier V to changes in the minimum power level value by the plate current caused by a predetermined minimum change in the equivalent load resistance across the tank circuit.

Upon closing the switch R3, the oscillator G becomes energized and upon supplying a minimum power to the load, it automatically connects the bridge amplifier V to its supply rectier by means of a responsive relay P3. The bridge arnplifler V thereupon energizes the follow-up control F and the variable coupler C is automatically adjusted to match the load impedance to the generator impedance.

As heating progresses in the reactive heater, the load impedance changes, and hence the plate current through the resistor P also changes. This change in the plate current through resistor P produces a change in the voltage impressed upon the calibrated potentiometer CP and unbalances the bridge amplier V. This causes the followup control F to be driven in the proper direction to cause a compensating change in the coupling when the plate current returns to its original value, the ybridge amplifier becomes rebalanced, the follow-up control is deenergized and the load impedance is thus rematched to the generator impedance.

The bridge amplier V is energized from the single-phase full wave rectiiier R2 in such a manner as to be independent of slight voltage variau tions of the source of power which supplies the variable power oscillator G. The three-phase full wave rectier Rl is not of the Voltage stabilized type, but the oscillator G is so designed that the plate current is driven into the saturation region of its plate voltage, plate current curve so that the plate or anode current through the resistor P is substantially constant for normally encountered changes in the value of the output voltage of the rectier. normal changes in the voltage or the rectifier cause a change in the plate currents through the resistor P. This change will be indicated by the wattrneter W.

The wattmeter W is energized from the resistor WI which also is calibrated in terms related to certain power values so that when properly adjusted the wattmeter will read directly the rate of power absorption in the material in the heater. If for any reason the reading of the wattmeter W does not agree with the setting of the calibrated potentiometer CP and that of the power level control GI, then adjustment of the potentiometer CP and control Gi may be made to secure the correct power value.

The power oscillator G is of the tuned plate, tuned grid type having the midpoint of its tank circuit grounded. The oscillator employs a pair of vacuum tubes G2 having their anodes connected adjacent opposite ends of the tank circuit coil G3 which is tuned by capacitor Gli.

The grids of the vacuum tubes G2 are interconnected by an inductor G5 having a midpoint connected to another inductor or choke coil G9. 'Ihe 1nductor G6 is connected to series reSSOI'S Greater than G1 and G8 which are connected to the cathodes of the vacuum tubes. The cathodes of the vacnum tubes are connected through the switch R3 to the negative side of the output of the threephase full wave rectifier Rl. The positive side of the output of the rectifier Ri is connected to the anode current resistor Pwhich in turn may be connected to a suitable ammeter and to ground.

The grid-to-cathode resistor G6 is adapted to be supplied with a biasing current from the power level control Gl which comprises an adjustable voltage or current rectifier unit which may be energized from the power lines L5 and Ll. The power level control Gl may be calibrated to indicate various load values or conditions in accordance with which the variable power oscillator G is to operate.

The resistor P connected between the positive terminal of the rectifier Ri and ground is provided with a radio frequency by-pass capacitor C3. From the juncture between the capacitor and the resistor, there is connected a resistor P2 which is arranged in series with the energizing coil of a relay P3 having a pair of normally open contacts V9.

The contacts Vt are connected in a series circuit extending from the positive side of the rectier R2 through cut out switch Vl l, a pair of contacts Fil of a manually operable switch Fg, a pair of contacts F3 of a manually operable switch Fl, through the relay contacts V9 to the adjustable arm on a resistor V5' connected beM tween resistors V5 and Vl. The resistors V5 and Vl together with the resistor VS are connected between the anodes of a pair of vacuum tubes V3 and Vd forming a part of the bridge amplifier V. The vacuum tube Vd has a cathode resistor VS provided with an adjustable contact which is connected to the grid of the tube. The cathode of the vacuum tube V3 is connected to a resistor Vl t.

The vacuum tubes V3 and Vd are arranged to operate relays FS and Fill. The relay F3 is connected to the anode of the vacuum tube V3 and to a unilaterally conductive device or rectier Fl l, to the anode or" the vacuum tube V4 and to the resistor Vl. The relay Fl@ is connected between the anode of the vacuum tube Vfl and another rectiiier Fl2 which is connected to the juncture between the resistor V5 and the anode of the vacuum tube V3.

The relay FS is provided with a pair of normally open contacts Fi which are connected in a series circuit extending from the line conductor L8 through two limit switches S2 and S3 to a reversible control motor FM which is arranged to drive the mechanism S through the gearing Si. The motor FM is a reversible alternating current motor, as, for example, a drag cup type having two inductive windings FM! and FM2 associated with capacitors FMS and FM4.

The juncture between the two inductive wind-- ings FM! and FMi is connected to the line conductor L9. The juncture between the two capacitors FMS and FM is connected through the `'limit switches S3 and S2 to the line conductor L8. The juncture between the capacitor Fil/lli 'and the inductor PM2 is connected to one of the contacts of the switch contact pair Fi and also to one contact of the switch contact pair F5. The pair of contacts F5 which are normally in open circuit position are a part of the manually operable switch Fl'. rihe juncture between the inductor FMS and the capacitor FME is connected to one contact of the pair of contacts F2 s'. forming a part of thel relay FHl-4 and also to` one of the pair4 of contacts Ff forming a part of theimanually operabl'eswitch F8.

For automatic operation of the apparatus Shown inl Figure 2 the material to' be heated is placed in thel reactive heater Dependent upon the nature and mass of the material and thev rate of heat generation desired a. particular power level setting isrequired. The cathode re'- sistor VITO' of the Vacuum tube V3' is provided with an adjustable contact-which is connected through asuitable inductor choke coil Vl2 to thel adjustable contact on the calibrated potentiometer CP'. Thel grid ofthe vacuum tube V3 is connected to the common juncture of the calibrated resistor CP and the anode current resistor P. The calibrated potentiometer CP is connected to ground and is so adjusted that a predetermined voltage therefrom may be applied to the grid-to-cathde circuit of the vacuum tube V3 in accordance with the required power level setting of the variable power adjustor GI. The power level control GI is thereupon adjusted to a graduation corresponding to the desired power level of operation for which the calibrated potentiometer CPl has been adjusted. Thereupon the switch. R3 is closed to supply anode potential to the variable power oscillatorY G.

When oscillation starts, current iiows through the resistor P2r and the relay coil P3 to close the relay contacts V9 thereby to complete a circuit between the vacuum tubes V3 and V4 and the positive terminal of the full wave rectier R2. Thus theY bridge circuit V is placed in operation and actuates the follow-up motor FM which, through the gearing SI, moves the reciprocable sleeve S between the limits provided for .by the location of the limit switches S2 and S3 arranged to be actuated` by the stop S4. The position of these switches is so selected that the safe limits of movement of the capacitor plates C are not exceeded.

The bridge amplifier V and the follow-up controll F thereupon automatically -adjust the impedance relation between the load and the generator so that they are matched. The voltage divider WI and the wattmeter W are so calibrated at the factory for diierent known load values matched to the generator that with the wiper of the divider WI being set ina given position, the wattmeter thereafter indicates the actual power absorption during the heating. operation.

As the heating process continues, the impedance relation between the loadand the power oscillator G will change. This change isrreected in a change of the anode or plate current supplied to the vacuum tubes G2 so that thevoltage drop' across the resistor P changes. This voltage drop will produce a change in the voltage appearing between the cathode and the grid. of the vacuum tube V3 soV as to unbalance the bridge amplifier thereby to bring about actuation of the follow-up motor FM which will adjust the coupling capacitor C until a matched relation is again established whereupon the motor FM becomes de-energized.

This rematching action takes place because of the operation of the bridge amplifier V which comprises the vacuum tubes V3 and V4 and the resistors V5, V6, and V1. At any instant the voltage between the grid and the cathode of the vacuum tube V3 is equal tothe sum of the voltage drop across the high potential portion of the calibrated potentiometer CP and the high potential portion of the cathode resistor VIII.

'ZL-heplate resistor'P was adjusted at they factory to* reflect, upon apredetermined change in the plate current,` suflicientY voltagey to bring: about operation of the bridge amplifier V. The. potentiometer GP` is calibrated in terms of power u-nitsso= that itmay' be adjustedA according: to the desired power level of operation of the oscillator G. When both the vacuum tubes V3 are in a balanced condition, the balance points VI4 and V2 are at equal potentials so that there is no voltagev difference across the relays F9 and FID and hence neither of these relays may operate.

Dependent upon the sign of the voltage difference between the balance points VI and V2, the properV relay coils F9 or F110 will be operated to bring about the proper direction of'rotati'on of thev follow-upl motor FM.

Iiir desiredthe apparatus shown in Figure 2 may beA operated manually or semi-automatically. For" thispurpose the manually operable switches VH, F1, and F8 are provided.

Opening of the switch VI I permits full manual operation by the use of` switches F1, and F8. Actuation of switch VII opens the power supply connections to the rectiiier R2 thereby rendering the bridge amplifier V inoperative. The resistor W'I of the wattmeter W is set to full sensitivity, and the heater being disconnected or empty, the power level control GI is adjusted to bring the needle of the wattmeter to the full scale mark O. Thereupon the heater is connected, or material placed therein, and the switch FT or F8 operated to change` the coupling C until the needle of the wattmeter is brought to the dial mark M so that the load is nowmatched to the generator. Thereupon the sensitivity potentiometer WI is adjusted to a point corresponding to dial mark M. The power level is then raised by actuation of the control GI to the desired value whereupon'the wattmeter needle will move above the mark M and read the true value of the power absorption.

ThereafterV the switches F1 and F8 are operated manually to keep the needle of the wattmeter' on the value mark to which it was raised on the setting of the power level by the control GI. If it is desired to indicate the maintenance of a matched condition` without the indication of power absorption, the sensitivity resistor WI may bev readjusted after rais-ing the power level to bring the Wattmeter needle back to the mark M.

For semi-automatic operation, the switch VII is left closed and the switches F1 and F8 operated manually to match initially the load to the generator. Thereafter the power level is raised by control Gl the reading of the wattmeter noted and potentiometer CP adjusted' to a corresponding position. Thereafter the bridge amplifier automatically maintains the matched relation as the load varies.

The semi-automatic operation is of particular advantage where work pieces of different sizes a-re to be treated in succession. For example, the apparatus may have a setting for a workpiece wherel the capacitor C is in a position to provide a large amount of capacitance. If thereupon` a different sized work piece is introduced into the heater H, the coupling may be too great so as to produce an overload which may result in tripping the circuit breakers. The operator of an apparatus of this type is aware of this possibility and'. hencel when. different sized work pieces are to be introduced, it isv only necessary to actuate either the switch-.Fl orl switch F8 tov change the; setting of the capacitor'C to decrease the coupling and the amount of energy which will be supplied to the load H. By opening the capacitor C, the initial load applied to the generator G will not be suflicient to produce an over.- load, and thereafter the automatic follow-up control may be relied upon to obtain the proper matching between the load and the generator,

The arrangement heretofore described has a bridge amplier V which is not subject to voltage variations caused by the change of power taken bythe rectifier Rl. The use of a power calibrated resistor CP for one of the tubes of the bridge amplifier eliminates any interconnection between the follow-up control and the high voltage and power level control connections of the variable power oscillator G.

While certain specific structural details have vbeen disclosed and described herein for the purpose of illustrating my invention, it will be apparent that other modiiications and changes may be made without departing from the spirit and scope of the appended claims.

This invention is hereby claimed as follows:

1. In radio frequency apparatus comprising a thermionic tube oscillator coupled to a variable reactive load circuit, means for controlling the ratio of coupling of Said load circuit to said os cillator, adjustable means for .presetting the power level `of operation of said oscillator, electric motor means for actuating said coupling Icontrol means, a motor control circuit connected to the anode circuit of said oscillator tube for rendering said motor means operable to actuate said coupling control in response to the anode current of said oscillator tube, said control circuit including adjustable means for predetermining the value of the anode current to which said control circuit responds and calibrated for adjustment according to the adjustment of said power level presetting means whereby said coupling control means is actuated by the motor means to establish and maintain a matched condition between the impedance of the reactive load circuit and the optimum oscillator load impedance value at each setting of the adjustable.,

power level presetting means.

2. In radio frequency apparatus comprising an oscillator having a space discharge tube, means for supplying direct current power to the plate circuit of said tube and a radio frequency power,

output circuit, a variable impedance load coupled to the radio frequency output circuit of the oscillator, a variable impedance device for changing the ratio of coupling of the load to the oscillator,

control means for actuating said device and cali-f to said device, and a motor control including space discharge tubes having their input circuits connected to the anode circuit of said oscillator and their outputs connected to control said motor for rendering said motor operative for operation in one direction on an increase in the anode current of the oscillator tube above a predetermined f value and for rendering the motor operative for operation in the opposite direction on a decrease in the anode current of said oscillator tube below y said predetermined value.

3. In radio frequency apparatus comprising an oscillator circuit including a space 'discharge tube,

a variable impedance load circuit coupled to the oscillator' circuit, a Variable impedance coupling control device in one of said circuits, a reversible motor for adjusting said device, a voltage responsive follow-up control circuit normally arranged to control the operation of said motor, adjustably presettable means for predetermining the power level of operation of said oscillator, an adjustable impedance in the plate circuit of said oscillator tube to supply control voltage for said follow-up control circuit to actuate the motor in one direction through the control circuit on an increase of platecurrent of said oscillator above a predetermined value and to actuate the motor through the control in the opposite direction on a decrease in said plate current -below said predetermined value, means for adjusting said voltage supply impedance in accordance with the presetting of said power level adjusting means thereby to pre-` set the predetermined value of plate current to which said follow-up control responds at each power level setting of the oscillator.

4. In radio frequency apparatus comprising an oscillator having a frequency determining resonant circuit including a variable impedance load having reactive and resistant components, said resonant circuit including a tank coil and a variable impedance coupling control device in said circuit between said tank coil and Said load, said oscillator including means for setting said oscil-llator for operation at diiferent selected power levels, said oscillator having a plate supply circuit including an adjustable impedance to supply a control voltage, a balanced bridge amplifier having its input connected to said adjustable impedance for response to said supply control voltage and having an output circuit, a reversible electric motor controlled by said output circuit mechanically connected to said variable impedance coupling control device, the input circuit of said balanced bridge amplifier having an adjustable impedance to cause said amplifier to respond to a given value of plate current through said adjustable impedance for each different power level setting of said oscillator whereby said coupling control device is adjusted to maintain the resistive component of the load substantially constant across the tank coil for each different power level setting of the oscillator. I

EUGENE MITTELMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,147,689 Chaffee Feb. 21, 1939 2,251,277 Hart et al. Aug, 5, 1941 2,252,941 Mittelmann Aug. 19,1941 2,324,525 Mittelmann July 20, 1943 2,338,412 Crandell Nov. 2, 1943 2,358,454 Goldstine Sept. 19, 1944 2,376,667 Cunningham et al. May 22, 1945 2,396,004 Gilbert Mar. 5,1946 2,415,799 Reifel et al. Feb. 11, 1947 2,416,172 Gregory et al Feb. 18, 1947 2,448,541 MaXson Sept. 7, 1948 2,467,285 Young et al Apr. 12, 1949 2,470,443 Mittelmann lViay 17, 1949 2,473,188 Albin June 14, 1949 OTHER REFERENCES 'Wireless World, Sept. 1944, pages 274-277. Mittelmann Load Rematching in Electronic Heating,i Electronics, February 1945, pages 

